This invention relates to feed forward circuits, and in particular, to a feed forward circuit that can serve as a feed forward isolator circuit or a feed forward filter circuit.
Electronically tunable bandpass filters that operate in the radio frequency range are generally obtained through tuning devices in which the reactance properties are a function of voltage or current. A varactor diode is an example of this type of tuning device although other types exist. The voltage or current applied to the tuning device is time variant when a radio frequency signal is present. In order to avoid excessive distortion, the ratio of the radio frequency signal to the bias signal that controls the reactance of the tuning device must be small. This restriction limits the application of radio frequency circuits that includes these devices to low signal levels, generally less than one watt. Additionally, a filter using this type of tuning device provides, due to the relatively low Q (the figure of merit of tuned circuits and filters), a high insertion loss to the radio frequency source to which the filter is connected.
An alternate approach in the prior art is to use electronically switched binary arrays of capacitors, inductors, or both to tune the circuit's passband frequency. The large number of tuning steps required for tuning these arrays makes them practical only for applications where the ratio of the tuning bandwidth to the tuned circuit's pass bandwidth is small.
Isolators of the prior art designs are generally limited to high frequencies (VHF and above) and to relatively narrow bandwidths (approximately 30%).